Système Authentification en Java à l'aide d'un fichier texte

"Mais si je veux qu'il y ai plusieurs users, et plusieurs password, je fais comment ?
"à chaque fois que je re-créé un compte, les dernières properties s'effacent"

import java.io.*;
import java.util.Properties;

public class UserCredentials {
    private final File credentialsFile;
    private final Properties properties;

    public UserCredentials(File credentialsFile) {
        this.credentialsFile = credentialsFile;
        properties = new Properties();
        if (credentialsFile.exists()) {
            readCredentialsFile();
        } else {
            writeCredentialsFile();
        }
    }

    private void readCredentialsFile(){
        try {
            properties.load(new FileInputStream(credentialsFile));
        } catch (IOException e) {
            throw new UncheckedIOException(e);
        }
    }

    private void writeCredentialsFile(){
        try {
            properties.store(new FileOutputStream(credentialsFile), null);
        } catch (IOException e) {
            throw new UncheckedIOException(e);
        }
    }

    public boolean userExists(String user) {
        return properties.containsKey(user);
    }

    public void setUserPassword(String user, String password) {
        properties.setProperty(user, hashPassword(user, password));
        writeCredentialsFile();
    }

    public boolean isUserPassword(String user, String password) {
        return hashPassword(user, password).equals(properties.getProperty(user));
    }

    public void deleteUser(String user) {
        properties.remove(user);
        writeCredentialsFile();
    }

    /**
     * TODO Ajouter une fonction de hachage pour ne pas enregistrer le password en clair dans le fichier
     * TODO Utiliser le user pour ajouter du salage au mot de passe
     */
    private String hashPassword(String user, String password) {
        return password;
    }
}

public static void main(String[] args) {
    UserCredentials userCredentials = new UserCredentials(new File("C:/userCredentials.properties"));

    System.out.println(userCredentials.userExists("KX"));  // false
    userCredentials.setUserPassword("KX", "helloWorld");
    System.out.println(userCredentials.userExists("KX")); // true
    System.out.println(userCredentials.isUserPassword("KX", "bonjour")); // false
    System.out.println(userCredentials.isUserPassword("KX", "helloWorld")); // true
    userCredentials.deleteUser("KX");
    System.out.println(userCredentials.userExists("KX"));  // false

    userCredentials.setUserPassword("toto", "passwordToto");
    userCredentials.setUserPassword("tata", "passwordTata");
    userCredentials.setUserPassword("titi", "passwordTiti");
}